3 Answers
3

In previous versions of MineCraft, a baby animal (or villager) had the same hitbox as a parent (even though they looked smaller). But, in version 1.5 this was fixed so babies are smaller than their parents.

As has been pointed out, pigs and chickens are already only 1 block tall, so an even smaller baby animal makes no difference.

However, adult cows, sheep, and villagers are taller than one block, allowing them to be filtered from the babies who can fit through a 1 block gap.